Skeletal bone defects of either the axial and craniofacial skeleton present formidable challenges to skeletal reconstructionists and to modern medicine. The mandible is a particularly difficult bone to repair and regenerate after surgical debridement of either neoplastic or inflammatory/infective lesions.
The surgical debridement of neoplastic masses of either primary or secondary metastatic tumours requires complex surgical procedures which are often unsuccessful in completely debriding the tumoral masses due to adhesion, metastatic growth and invasion into surrounding tissues, in particular vascular tissue. Unsuccessful debridement of neoplastic tumours leads to further secondary masses growth, invasion and metastatic tumoral growth with ultimate death.
Bone regeneration in clinical contexts requires three key components: an osteoinductive signal, a suitable substratum with which the signal is to be delivered and which acts as a scaffold for new bone to form, and host responding cells capable of differentiation into bone cells as a response to the osteoinductive signal. The signals responsible for osteoinduction are proteins collectively called the bone morphogenetic and osteogenic proteins (BMPs/OPs). BMPs/OPs are forming growth factor-β supergene family (TGF-β). The superfamily also includes four TGF-β isoforms, the transforming growth factor-β family per se [ref. 1-3 for reviews]. Members of both BMP/OP and TGF-β families are pleiotropic factors that, have potent and diverse effects on cell proliferation, differentiation, motility and matrix synthesis [1-3].
The three mammalian TGF-β isoforms (TGF-β1, β2 and β3) share limited homology with members of the BMP/OP family (BMP-2 through BMP-6 and osteogenic protein-1 and -2 [OP-1 and OP-2]) [1-3]. A striking and discriminatory feature of the BMPs/OPs is their ability to induce de novo cartilage and bone formation in extraskeletal heterotopic sites of a variety of animal models. Recombinant human (h) BMP-2, BMP-4 and OP-1 (also known as BMP-7) singly initiate endochondral bone formation in the subcutaneous space of the rat [1-3].
On the other hand, the TGF-β isoforms, either purified from natural sources or expressed by recombinant techniques, do not initiate endochondral bone formation in the in vivo bioassay in rodents [3-6].
Since TGF-β isoforms are most abundant in the extracellular matrix of bone as well as in many other tissues [3,4] and that the isoforms synergise in inducing large ossicles in the primate [1,3,7,8], the applicant envisages that the use of TGF-β isoforms in conjunction with a physiologically acceptable delivery vehicle is of paramount importance for inducing new bone formation in primates including man. Indeed, although BMPs/OPs can initiate bone formation following a single local application, the generation of new bone may not be rapid, and furthermore, substantial amounts of recombinantly produced BMPs/OPs may be required to achieve the desired effect in terms of bone volume and bone mass at site of skeletal defects.
Studies performed in rodents have shown that the TGF-β isoforms do not initiate bone formation when implanted in heterotopic extraskeletal sites [3-6]. In marked contrast, the applicant has shown that TGF-β1 and TGF-β2 induce endochondral bone formation when implanted heterotopically in the rectus abdominis muscle of adult primates of the genus Papio [3,7,8,9]. In calvarial defects, a site-specificity of induction of TGF-β1 and TGF-β2 has been found, however [3,9,10], i.e. with limited bone induction in calvarial defects and florid endochondral bone formation heterotopically in the rectus abdominis muscle of the primate Papio ursinus. In the same animal and implanting identical doses of TGF-β1 or TGF-β2, bone induction is florid in the rectus abdominis muscle but limited in calvarial defects [3,9,10,11].
This observed site and tissue-specificity of TGF-β isoforms in different tissue sites, i.e. the calvarium and the rectus abdominis muscle, may be explained by the paucity of TGF-β responding cells at the site of orthotopic calvarial implantation and/or by an increase expression of Smad-6 and Smad-7 gene products in calvarial sites down regulating the activity of the implanted TGF-β proteins [9, 11].